Separation of radium and rare earth elements from monazite

ABSTRACT

A method of chemically extracting radium-228, rare earth metals, thorium, the decay products of thorium, and phosphates from thorium-containing ores. The method involves breaking thorium-containing ore into fragments, wetting the fragments with a concentrated strong acid to make a slurry, heating the slurry, passing the heated solution through a first anion exchange column, retaining metals and radium-228 captured on the resin, allowing the radium-228 ions to decay to actinium-228, purifying the actinium-228 fraction, sending the actinium-228 fraction through a capture column, eluting the captured thorium-228 with acid, removing radium from the solution, retaining the radium-228 fraction for isomer in-growth, retaining decay products from the radium-228, separating the REEs from the process stream; and eluting and retaining the REEs.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to a novel method of chemicalextraction fashioned to extract and separate rare earth metals, thorium,its decay products, and phosphates from thorium-containing ores. Thenovel process efficiently recovers phosphoric acid, radium-228,radium-228's decay sequence, including its isomers, actinium-228 andthorium-228, and the rare earth elements including lanthanum, cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium ytterbium, lutetium,yttrium and scandium from various thorium containing head ores or feedmaterials including monazite and bastnaesite.

2. Background Art

Radium-228 is present in small quantities, averaging approximately 3.5grams per thousand tons of refined monazite sand. Radium-228 is a highlyvaluable material for medical applications as some of its natural orartificial decay products can be used as alpha emitting components intargeted alpha therapy. Monazite is the heavy sand material from whichthorium oxide and the rare earth elements are commonly commerciallyproduced. Thorite is thorium silicate. Bastnaesite is a carbonatitesource material for rare earth elements (“REE”).

Rare earth elements are in increasing industrial demand. Manyelectronics related applications need lower cost REE elements. The REEare used in making superior lasers, stronger magnets, special glasses,various semiconductors, light emitting materials, phosphors, and thelike. Radioactive decay products from thorium-containing materials,including monazite mining tailings, add cost to monazite miningoperations.

Thorium (element 90 in the periodic table) is mildly radioactive.Thorium is widely distributed in nature with an approximate averageconcentration of 10 ppm in the lithosphere. It is present in the earth'scrust in association with many phosphates, silicates, carbonates andoxide minerals. Thorium occurs in association with uranium and the rareearth elements, the lanthanides, many types of rock as veins of thorite,thoranite, uranothorite, and as monazite in granites. Monazite is amixed phosphate mineral with the general chemical formula (REE/Th/U)POsub.4. Monazite is a major source for REE and a secondary source forphosphate, thorium and uranium. Phosphate mine tailings, uranium andrare earth mine tailings contain radioactive thorium. For the most partthis radioactivity is from the first decay product of thorium-232, whichis radium-228, and from the decay sequence starting with radium-228.

Because thorium is radioactive, thorium is a regulated material and mustbe stored and accounted for by mine operator under strict international,national and local standards and set out in a locally issued miningpermit. This stewardship cost for the rare earth, thorium, and phosphateextractive industries is a significant commercial expenditure. Eachtailing dump, or other location where thorium-containing materialsaccumulate, must be accounted for as radioactive waste. Becausethorium-containing materials are classified as low level radioactivewaste, potential third party environmental, personal injury and propertyclaims against the thorium possessor may ripen over time, exposing thepossessor to liability and damages. These property and injury claims aretypically excluded from insurance coverage under the mining operators'and mine owners' insurance contracts.

DISCLOSURE OF INVENTION

This invention discloses means and methods of a novel chemicalseparations process that more efficiently recovers and separates theeconomically valuable elements, phosphoric acid, the REE in chemicallypurer form, and separates the radium-228, the radioactive “mesothorium,”from the worked material.

The present invention removes radium-228 from the thorium-containingmaterials produced in the monazite mining waste stream to temporarilyreduce the radioactivity of stockpiled thorium-containing materials. Theprocess permits neutralized thorium to be stockpiled so that it can berecovered in the future for its nuclear energy content or laterre-milked for its radium-228 content. Because the radioactive emissionsfrom the thorium containing tailings are significantly reduced after theradium-228 is separated and removed, the remaining neutralized thoriumin the tailings or stockpile will have significantly lower radioactivityuntil the natural decay of in the thorium replaces the radium-228 andthe physical equilibrium of the decaying isotopes is restored. Theprocess also removes the valuable REE so that these can be efficientlyseparated into pure metals or pure compounds. Finally, to the extent theore contains phosphates, the inventive extraction process recovers thephosphates as phosphoric acid.

The inventive chemical separation process has the advantage that asradium-228 is collected, phosphoric acid is also collected, and each ofthe rare earth elements is collected in a highly purified form. Theradium-228 is useful for medical isotope applications. When it isseparated from thorium, thorium emissions are reduced so that in somejurisdictions it would be classified as the least regulated naturallyoccurring radioactive materials.

Referring to FIGS. 1-1B and FIG. 2, we can see that the half life ofthorium is approximately the age of the universe, 15 billion years orso. The half-life of thorium's first decay product, radium-228 iscomparatively short: 5¾ years. The other decay products have shorterhalf lives. Radium-228 decays to actinium-228. Actinium-228 has a halflife of 6.15 hours. Actinium-228 decays to thorium-228 with a half lifeof 1.91 years. Thorium-228 decays to radium 224 with a 3.66 day halflife, which decays to radon-220 with a 55.6 second half life, whichdecays to polonium-216 with a 0.15 second half life, which decays tolead-212 with a 10.64 hour half life that decays to bismuth-212 with aone hour half life that branches with the ultimate end product beingstable lead-208.

It is the radioactivity of thorium's decay daughters and not the parentthorium-232 isotope that imposes the regulatory and stewardship costs onoperators having thorium in feed material and tailings. The regulatorycosts can be reduced significantly by the present invention because theradioactive disintegrations of concern from the thorium-containingmaterials are mostly from thorium's comparatively short lived decayproducts, commencing with radium-228. After radium-228 is removed fromthe process stream, thorium-containing tailings will include lessmaterial ionizing from the decays of the radium-228 in the decay chain.Instead, radium-228, already separated and concentrated, will beavailable for highly valuable medical isotope applications for thetreatment of diseases.

The separation of radium-228 from thorium-232 reduces potentialliabilities associated with the possession of thorium for the mineoperator for the period of time before sufficient new radium-228 “growsin” the thorium by the inexorable alpha decay. Immediately after theseparations the remaining thorium-232 will have lower activity becausesignificant quantities of radium-228 will have been removed. Theradioactivity from radium-228 gradually returns in the thorium-232, buta mine operator will have sufficient time to permanently bury orotherwise properly dispose of the thorium-containing materials duringthe period of time when the rate of treated thorium-232 disintegrationsis reduced, perhaps below the applicable regulatory threshold in somejurisdictions.

Other novel features which are characteristic of the invention, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing, in which preferred embodiments of the inventionare illustrated by way of example. It is to be expressly understood,however, that the drawing is for illustration and description only andis not intended as a definition of the limits of the invention. Thevarious features of novelty which characterize the invention are pointedout with particularity in the claims annexed to and forming part of thisdisclosure. The invention resides not in any one of these features takenalone, but rather in the particular combination of all of its structuresfor the functions specified.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those setforth above will become apparent when consideration is given to thefollowing detailed description thereof. Such description makes referenceto the annexed drawings wherein:

FIGS. 1-1B comprise a chart 100 showing the short-lived decay chains ofthorium-232 and thorium-238, as well as the decay chains of uranium-238,uranium-234, and uranium-235;

FIG. 2 is a table 200 showing the natural decay products of thorium; and

FIG. 3 is a schematic flow chart showing the method steps of theindustrial chemical separation process of the present invention.

Various method steps and operational features may change in practice.The details concerning the precise composition of the production resins,columns, filters, solvents and catalysts will vary depending on the sizeand volume of the production facility, and the degree of purity soughtfor the rare earth elements, the phosphates, the thorium and theradium-228 recovered. Modifications and changes will be driven bypractical and economic concerns and according to the most prudentpractices in view of the characteristics of the monazite or other feedmaterial supplied to the plant.

BEST MODE FOR CARRYING OUT THE INVENTION

The chemical separation process summarily described above is now set outfully for monazite. A similar approach works for bastnaesite and otherthorium-containing ores. The monazite sand is first conventionallyconcentrated. This is accomplished by separating the uniform particlesize grains based on physical properties, specific gravity, magneticsusceptibility, electrical conductivity, and surface properties. Themonazite-containing sand is processed and conductive ilmenite andconductive rutile constituents are removed. Non-conducting monazite,which is heavy and moderately magnetic, is isolated from non-magneticcomponents and other materials remaining in the feed material. Theresulting concentrate having been processed by magnetic and electricalmeans is generally more than 90% monazite. This concentrated monazitesand is finely ground and is ready for digestion by the inventiveprocess described herein.

Monazite is a phosphate ore. It occurs in three varieties: (Ce La Nd ThY) POsub.4, (La Ce Nd) POsub.4 and Nd, La Ce)POsub.4. Its generalformula is (REE)Th U POsub.4. Its economically significant mineralsinclude the REE group metals, phosphate, thorium and radium-228. All ofthese are extracted by the novel method disclosed herein.

Referring now to FIG. 3, the first step 300 in processing monazite underthe present invention is to pulverize concentrated monazite by passingit through a 200 mesh screen (0.074 millimeters) or through largerdimensions of up to 10 mesh (2.000 millimeters). Pulverized monazitesand or other thorium-containing ore is then first wetted with 8M nitricacid 310 to make a slurry, which is then heated. This solution isfiltered and passed over an anion exchange resin 320. The solutionpassing through the first column contains (non-retained) REE ions,actinium ions and radium ions 330. Several metals are retained 340 onthe resin in the first column, including Th, Fe, Co Ni, Cu Ag Sn Zn CeA. Sc Te Zr Hf Cr Mo Mn and U 350. The metals retained on this columncan be removed and recovered 360 using a 90% methanol-10% nitric acidsolution. Thorium may be eluted from the first resin column's resinusing 1 M HNOsub.3.

After passing through the first column, the liquid solution containsREE, actinium and radium ions in the nitrate solution. The radium ionsare allowed to decay 370 to Actinium-228, as actinium-228 is the firstdecay product of radium-228,which has a 100 hour maximum life (this isthe time needed for all the separated actinium-228 to entirely decay).The purified actinium-228 fraction is sent through a final thorium-228capture column 380. Thorium-228 390 is eluted with 1M nitric acid andcan be further purified as needed by passing the solution overadditional anion exchange resin columns (not shown in the flow diagram).

Next, radium is removed from the solution via co-precipitation withbarium nitrate 400. Secondary and tertiary co-precipitations of bariumnitrate ensure complete separation of the radium fraction. This fraction410 is withheld for “isomer in-growth.” The decay products from theradium-228 are retained for use as the alpha-emitting material thatultimately finds use in medical isotope “generators”.

The solution 420 containing the REE, after the radium-228, thorium andother metals are separated, is ideal for the production of highly pureforms of the rare earth elements. In a preferred method, the REE areseparated 430 according to the size of the rare earth metal ions, withthe smallest being separated first and the largest last. The detailedsteps for separating the rare earth lanthanides of the preferredtechnique include procedures using reversed-phased partitionchromatograph. This technique for separating the rare earths has gainedincreasing importance in recent years. This is because the separationfactors between adjacent rare earths elements are in several casesbetter when extracting these elements with organic phosphorous compoundsthan when eluting from cation exchange resins in the presence of organiccomplexing agents.

The liquid is then passed through a stationary phase 440. The organicphosphorous compounds that are most frequently used as stationary phasesare bis(2-ethylhexyl)-o-phosphoric acid (HDEHP) and tri-n-butylphosphate(TBP) Also, bis(di-n-hexyl-phosphinyl)methane (HDPM) anddi-n-butylphosphate have been recommended for this purpose. Also,long-chain amines, e.g. trioctylamine and dinonylnaphalenesulphonic acidin heptane, have been employed as stationary phases.

The following substances can be used as supports for these stationaryphases: Corvic (poly(vinyl chloride-vinyl acetate) co-polymers),siliconized kieselguhy or silica gel, Kel-F (polychlorotrifluoroethane),or filter paper. The mobile phases are mostly pure aqueous solutionscontaining acids, such as nitric, hydrochloric or perchloric acids.

The best separations factors (about 2 to 5) between adjacent rare earthsare obtained with HDEHP as the stationary phase. These are in mostcases, better than in cation exchange systems when usingα-hydroxyisobutyrate or lactate as eluting agents for the rare earths.TBP is less suitable, although the separation factors (about 2) that areobtained with this extraction are higher than in adsorption on cationexchange resins from citrate, glycollate or lactate solutions.

Irrespective of the kind of stationary phase that is employed, the rareearth elements are eluted in the order of increasing atomic number, thatis, lanthanum first and lutetium last. This is the reverse order ofelution as observed when using cation exchange chromatography. It isalso the order of increasing partition coefficient for the lanthanides,when portioning between aqueous mineral acid solutions and a solution ofHDEHP, TBP, etc. in an inert solvent at constant acidity andconcentration of extractant.

Besides the higher separation factors there are other advantages ofthese reverse-phase techniques over the conventional methods usingcation exchange in the presence of complexing agents. One advantage isthat the elution curves of the rare earths obtainable by reversed-phasepartition chromatography are narrow, quite symmetrical and show notailing. Furthermore, the effluent from the column does not contain anysalts or complexing agents but only mineral acids which can be readilyremoved by evaporation. Also, the procedure is not time consuming and itmay be carried out satisfactorily at room temperature.

The recovered products from the process stream include thorium-232, rareearth elements, scandium, yttrium, radium-228, actinium-228, thorium-228(by decay) 450, and phosphoric acid 460. A distillation of nitric acid470 can be effected to introduce the distillate back in the processstream for use at steps 310 and 430.

Thorium-232, now depleted of radium-228, can be reserved and used fornuclear fuel or other applications. When stripped of its decay productsthorium-232 can be of high commercial value as a catalyst or as analloying metal. The Rare Earth Elements can be marketed and sold inhighly pure form. Value is added by the ability of this chemicalseparations process to produce pure rare earth elements. Other forms ofchemical separation methods for the rare earth elements are moreexpensive because all of the rare earth elements have similar oridentical chemical properties. High purity rare earth elements areindustrial commodity materials in demand that have growing applicationsto enhance magnetic performance, to enhance semi-conductor performance,to enhance phosphor performance, to enhance diode performance and toprovide for photon to copper and back again transducers. Rare earthelements can be more economically produced by this disclosure frommonazite and other REE ores because the process contemplates thesegregation of high purity rare earth metals.

Importantly, radium-228, actinium-228, and their decay productthorium-228, are concentrated and recovered by the techniques disclosed.These materials can be used as generators of valuable medical isotopematerials. The three isomers, radium-228, actinium-228, and thorium-228,can be irradiated either with high energy neutrons for n, 2n reactionsor with high energy gamma photons for gamma, n reactions or simplyreserved for harvest of useful decay products. For example radium-227can be produced by gamma, n on radium-228. Radium-227 has a 42.2 minutehalf life and its immediate decay product is actinium-227 that has ahalf life of 21.773 years. Actinium-227 can be used to supply the highlysought medical isotope thorium-227 having a half life of 18.72 days thatdecays to radium-223 useful for the treatment of bone cancers and forpain palliation. This disclosure provides means to collect valuableprecursor isotopes with uses in the treatment of cancer along withvaluable rare earth metals and commercial phosphate containingmaterials.

Alpha particles from short-lived alpha emitters in the natural thoriumdecay chain provides superior treatment options for many diseases. Highz alpha emitters from thorium-containing ores can be linked withmolecules that seek and bind to abnormal cells. For many cancertreatments targeted alpha therapy provides better outcomes thanchemotherapy, gamma radiation therapy, x-ray therapy or beta radiationtherapy, as are presently practiced.

The inventive method disclosed herein further teaches the importantsteps and novel techniques to increase the purity of rare earth metalsproduced from monazite and other ores, it provides means to reduce theradioactivity of thorium produced from monazite and other ores, and itprovides the means to recover radium-228 to provide a reliable source ofisotopes useful in the treatment several types of cancer and otherinfectious diseases.

The foregoing disclosure is sufficient to enable those with skill in therelevant art to practice the invention without undue experimentation.The disclosure further provides the best mode of practicing theinvention now contemplated by the inventor. While the particularchemical separation method herein shown and disclosed in detail is fullycapable of attaining the objects and providing the advantages statedherein, it is to be understood that it is merely illustrative of thepresently preferred embodiment of the invention and that no limitationsare intended concerning the detail of process steps other than asdefined in the appended claims. Accordingly, the proper scope of thepresent invention should be determined only by the broadestinterpretation of the appended claims so as to encompass obviousmodifications as well as all relationships equivalent to thoseillustrated in the drawings and described in the specification.

1. A chemical separation process for removing radium-228, from materialscontaining thorium-232 to make “neutralized thorium”, and to separaterare earth elements from common thorium ores or mining waste streams,comprising the steps of: (a) breaking concentrated monazite or otherthorium-containing ore into small fragments; (b) wetting the fragmentswith a concentrated strong acid to make a slurry, wherein the slurrycontains, among other things, rare earth elements (REE), actinium andradium ions; (c) heating the slurry made in step (b); (d) filtering andpassing the heated solution through a first anion exchange column havingan anion exchange resin; (e) retaining metals on the resin the firstanion exchange column; (f) allowing the radium ions to decay toactinium-228; (g) purifying the actinium-228 fraction; (h) sending theactinium-228 fraction through a final thorium-228 capture column; (i)eluting the captured thorium-228 with 1M HNOsub.3; (j) removing radiumfrom the solution; (k) retaining the radium-228 fraction for isotopein-growth; (l) retaining decay products from the radium-228 fraction foruse as alpha-emitter used in medical isotope generators; (m) separatingthe each individual REEs from the process stream; and (n) eluting andretaining the separated REEs.
 2. The process of claim 1, wherein step(a) comprises pulverizing or comminuting.
 3. The process of claim 2,wherein step (a) comprises passing the concentrated monazite or otherthorium-containing ore through a mesh screen.
 4. The process of claim 4,wherein the mesh screen used in step (a) is between 10 and 200 mesh. 5.The process of claim 1, wherein the strong acid used in step (b) is 8Mnitric acid.
 6. The process of claim 1, further including the step ofremoving and recovering the metals retained in step (e) by using a 90%methanol-10% nitric acid solution.
 7. The process of claim 6, whereinthe metals retained include, Th, Fe, Co Ni, Cu Ag Sn Zn Ce A. Sc Te ZrHf Cr Mo Mn and U.
 8. The process of claim 1, further including the stepof eluting thorium from the resin in the first anion exchange columnusing 1 M HNOsub.3.
 9. The process of claim 8, further including thestep of further purifying the captured thorium-228.
 10. The process ofclaim 1, further including the step of passing the solution over atleast one additional anion exchange resin column.
 11. The process ofclaim 1, wherein step (j) involves removing radium from the solution viaco-precipitation with barium nitrate
 12. The process of claim 11,further including the step of secondarily and tertiary co-precipitatingbarium nitrate to ensure complete separation of the radium-228 fraction.13. The process of claim 1, wherein step (m) involves separating eachREE in order of the size of the rare earth metal ions, with the smallestseparated first and the largest last.
 14. The process of claim 1,wherein step (m) involves separating each REE lanthanides usingreversed-phased partition chromatograph.
 15. The process of claim 14,wherein the stationary phase of the reversed-phased partitionchromatograph employs an organic phosphorous compound.
 16. The processof claim 15, wherein the organic phosphorous compound of the stationaryphase is selected from the group consisting ofbis-(2-ethylhexyl)-o-phosphoric acid (HDEHP), and tri-n-butylphosphate(TBP), bis(di-n-hexyl-phosphinyl)methane (HDPM), anddi-n-butylphosphate.
 17. The process of claim 14, wherein the stationaryphase uses a long-chain amine.
 18. The process of claim 17, wherein thelong-chain amine is selected from the group consisting oftri-octyl-amine and di-nonyl-naphalene-sulphonic acid in heptane. 19.The process of claim 14, further including the step of using a supportfor the stationary phase.
 20. The process of claim 19, wherein thesupport is selected from the group consisting of Corvic (poly(vinylchloride-vinyl acetate) co-polymers), siliconized kieselguhy or silicagel, Kel-F, (polychlorotrifluoroethane), and filter paper.
 21. Theprocess of claim 14, wherein the mobile phases are substantially pureaqueous solutions containing strong acids.
 22. The process of claim 21,wherein the strong acids are selected from the group consisting ofnitric acid, hydrochloric acid, and perchloric acid.
 23. The process ofclaim 1, wherein step (n) involves eluting each REE in order ofincreasing atomic number.
 24. The process of claim 1, further includingthe step of recovering phosphates as phosphoric acid.
 25. The process ofclaim 1, further including the step of distilling nitric acid from theprocess stream and placing the distillate back in the process stream foruse in an earlier method step.
 26. The process of claim 1, furtherincluding the step of reserving thorium-232 depleted of radium-228.